1. Field of the Invention
The present invention relates generally to a code generator for a CDMA mobile communication system, and in particular, to a TFCI (Transport Format Combination Indicator) code generator and a method for embodying the same.
2. Description of the Related Art
An IMT-2000 system, a future CDMA mobile communication system, transmits various service frames for supporting a voice service, an image service, and a data service within one physical channel. The service frames are transmitted at either a fixed data rate or a variable data rate. The different services transmitted at the fixed data rate are not required to separately notify a spreading rate to a receiver. However, the services transmitted at the variable data rate must inform the receiver of the spreading rates of the respective service frames, since the data rate may be changed during the services. The spreading rate is determined depending on the data rate.
In the IMT-2000 system, the data rate is in inverse proportion to the data-spreading rate. When the frames used by the respective services have different data rates, a TFCI (Transport Formation Combination Indicator) bit is used to indicate a combination of the currently transmitted services. The TFCI enables correct reception of the services.
FIG. 1 illustrates a method of using the TFCI in an NB-TDD (Narrow Band-Time Division Duplex) system, by way of example. In particular, the NB-TDD system uses 8PSK (8-ary Phase Shift Keying) modulation for high-speed data transmission, and codes the TFCI value with a length=24 code before transmission.
Referring to FIG. 1, one frame is comprised of two subframes. The subframes each include 7 time slots TS#0-TS#6, a downlink pilot time slot DwPTS, a guard period where no signal is transmitted, and an uplink pilot time slot UpPTS. The 7 time slots TS#0-TS#6 are divided into downlink time slots TS#0, TS#4, TS#5 and TS#6, and uplink time slots TS#1, TS#2 and TS#3. Each time slot comprises data fields for storing data symbols, two TFCI fields for storing TFCIs associated with the data symbols stored in the data fields, a field for storing a midamble, a field for storing SS symbols, and a field for storing TPC (Transmission Power Control) symbols. A time length of the frame is Tf=10 ms, and a time length of the subframe is Tsf=5 ms. In addition, a time length of each time slot is Tslot=0.625 ms.
FIG. 2 illustrates a structure of a transmitter in the conventional NB-TDD CDMA mobile communication system. Referring to FIG. 2, a TFCI encoder 200 encodes input TFCI bits at a given coding rate and generates coded TFCI symbols. The coded TFCI symbols are provided to a first multiplexer (MUX) 210 as one input. At the same time, other signals comprised of the data symbols, the SS symbols and the TPC symbols included in one slot of FIG. 1 are provided to the first multiplexer 210 as another input. The coded TFCI symbols, the data symbols, the SS symbols, and the TPC symbols are multiplexed by the first multiplexer 210. The multiplexed signals are then channel-spread with an orthogonal code by a channel spreader 220. The channel spread-signal signals are scrambled with a scrambling code by a scrambler 230, and then provided to a second multiplexer 240 as one input. At the same time, a midamble signal is provided to the second multiplexer 240 as another input, and multiplexed with the scrambled signals. As a result, the second multiplexer 240 outputs a signal having the slot format shown in FIG. 1. The first and second multiplexers 210 and 240 output the frame format of FIG. 1, under the control of a controller (not shown).
FIG. 3 illustrates a structure of a conventional NB-TDD receiver corresponding to the above-described transmitter. Referring to FIG. 3, a signal received from the transmitter is demultiplexed by a first demultiplexer (DEMUX) 340, so that a midamble signal is separated from the received signal. The midamble-removed signal is descrambled by a descrambler 330 with the scrambling code used by the transmitter. The descrambled signal is channel-despread by a channel despreader 320 with the orthogonal code used by the transmitter. The despread signal is demultiplexed (separated) into coded TFCI symbols and other signals by a second demultiplexer 310. The “other signals” refers to the data symbols, the SS symbols and the TPC symbols. The separated coded TFCI symbols are decoded into TFCI bits by a TFCI decoder 300.
The TFCI bits indicate 2 to 4 combinations expressed with 1 to 2 bits according to combination of transmission information, and default TFCI bits indicate 8 to 32 combinations expressed with 3 to 5 bits. In addition, extended TFCI bits indicate 64 to 1024 combinations expressed with 6 to 10 bits. The TFCI bits are required information when the receiver analyzes transmission information of the received frames. Therefore, if a transmission error occurs in the TFCI bits, the receiver may not correctly receive the respective service frames. For this reason, the TFCI bits are encoded at the receiver using a high-efficiency error correcting code capable of correcting a possible transmission error.
FIG. 4 illustrates an error correction encoding scheme for a 5-bit default TFCI. In particular, FIG. 4 illustrates a structure of a (24,5) encoder by way of example. That is, the drawing shows a scheme for outputting a 24-symbol coded TFCI by encoding a 5-bit default TFCI.
Referring to FIG. 4, a (16,5) bi-orthogonal code encoder 400 encodes 5-bit TFCI input information into a 16-symbol coded TFCI, and provides the 16-symbol coded TFCI to a repeater 410. The repeater 410 outputs the intact even-numbered symbols out of the provided coded TFCI symbols, and repeats the odd-numbered symbols, thus outputting a total of 24 coded TFCI symbols. Herein, the scheme has been described with reference to the 5-bit input TFCI. However, when the input TFCI is comprised of less than 5 bits, a zero (0) bit(s) is added at the head of the input TFCI to make a TFCI having a length of 5 bits.
An intercode minimum distance of the (16,5) bi-orthogonal encoder 400 is 8. In addition, even the (24,5) code output from the repeater 410 also has the minimum distance of 8. In general, an error correcting capability of binary linear codes depends on the intercode minimum distance of the binary linear codes. A reference, An Updated Table of Minimum-Distance Bounds for Binary Linear Codes (A. E. Brouwer and Tom Verhoeff, IEEE Transactions on information Theory, VOL 39, NO. 2, MARCH 1993), discloses an intercede minimum distance which depends on the input and output values of the binary liner codes to be optimal codes depending on the number of coded symbols generated by encoding input information bits.
Taking into consideration the fact that the TFCI transmitted in FIG. 4 is comprised of 5 bits and the coded TFCI is comprised of 24 symbols, the intercede minimum distance required in the above-stated reference is 12. However, since the minimum distance between the coded symbols output from the encoder shown in FIG. 4 is 8, the encoder does not have the optimal codes. If the error correction encoding scheme of FIG. 4 fails to have the optimal codes, an error rate of the TFCI bits increases in the same channel environment. As a result, the receiver may erroneously recognize a data rate of the data frames, increasing frame error rate (FER). Therefore, there is a demand for an error correction encoding scheme capable of obtaining optimal codes through encoding of the TFCI bits.